docs: def rdf_source(self, format="turtle"):
          """
          Wrapper for rdflib serializer method.
          Valid options are: xml, n3, turtle, nt, pretty-xml, json-ld [trix not working out of the box]
          """
          s = self.rdflib_graph.serialize(format=format)
          if isinstance(s, bytes):
              s = s.decode('utf-8')
          return s